1. Field of the Invention
The present invention relates to a cardiovascular graft and a method of forming same. More particularly, the invention relates to a cardiovascular graft fabricated of a porous, biocompatible polymer system which provides for cellular ingrowth and/or increased flexibility. The method of forming the graft utilizes a non-solvent, two component, hydrophobic polymer system.
2. Description of the Prior Art
Heretofore various porous graft structures and methods of forming same have been proposed.
Two such graft structures and methods for their formation are disclosed in the following U.S. Patents:
______________________________________ U.S. Pat. No. PATENTEE ______________________________________ 4,334,327 Lyman 4,355,426 MacGregor ______________________________________
The Lyman U.S. Pat. No. 4,334,327 discloses a flexible ureteral prosthesis (graft) fabricated from copolyurethane materials. The prosthesis includes an elongate duct having a lumen whose interior surface is ultrasmooth to impede incrustation. An external cuff, formed of a foam-like material, is formed around a portion of the elongate duct. The cuff has at least 40% void space therein which provides a proper density for receiving sutures to enable fixation of the cuff by suturing to appropriate muscular tissue. The inner diameter of the prosthesis must be conformed to the outer diameter of the ureter of the recipient. Further, the prosthesis is provided with a one-way valve to prevent backflow of urine.
The process of formation of the prosthesis involves selecting a tubular mandrel having a highly polished surface and an outer diameter corresponding to a desired inner diameter for the prosthesis. One end of the mandrel is configured to conform to a cavity shape which is of acceptable size for forming the body of the one-way valve.
The next step in this process involves applying a fluid layer of block copolymer to the mandrel. The block copolymers found particularly suitable as ureter replacement materials include copolyurethanes, copolyether-urethanes and/or copolyether-urethane-ureas. With the mandrel suitably coated, the copolymer layer is solidified to fix its shape into conformance with the mandrel surface. Mold blocks are than secured around terminal segments at each end of the coated mandrel to form boundaries for the formation of an exterior cuff. These blocks have an opening centrally disposed therein corresponding to an approximate diameter of the coated mandrel to facilitate mounting of the mandrel within the respective mold blocks. The cuff is then formed by permanently affixing a material whose final state develops a foam-like composition over the coated mandrel between the mold blocks. Once the cuff is formed and appropriately configured to facilitate suturing to fascia within the patient, the mold blocks are removed and the mandrel withdrawn. The foam like end product structure may be fabricated by admixing powdered inorganic salt to a solution of approximately 12% to 17% (w) block copolymer or may utilize a fluid transfer method for establishing the voids throughout the cuff material.
The MacGregor U.S. Pat. No. 4,355,426 discloses a cardiovascular prosthetic device comprising a porous surface and a network of interconnected interstitial pores below the surface of the device in fluid flow communication with the surface pores.
Several other devices are disclosed which fall broadly into two classes, rigid items and flexible polymeric items.
The flexible porous polymeric grafts are formed from a segmented polyurethane and more preferably a segmented hydrophilic polyurethane. The graft may be provided with a porous surface and subsurface network on a coherent substrate or may be formed as a wholly porous structure.
Various specific structural embodiments of flexible graft are dependent upon the function the graft is to serve, as disclosed in the MacGregor patent.
Further, MacGregor proposes several different procedures for forming the various graft structures defined, all of which require the use of a polymer resin and a solvent.
As described above, the prior methodology of formation of graft structures has involved the mixing of water soluble inorganic salts into polymer-solvent systems and then forming a graft of a desired but limited thickness by one of many procedures available. The resulting polymer network is then cured and leached of salt by soaking in an aqueous solution.
Also, foaming agents and blowing agents have been used to produce "pseudo-porous grafts", i.e., to produce a closed pore cellular structure to the graft. The pore sizes are often irregular and difficult to control and can be larger than the 200 micron maximum size recommended for tissue ingrowth. Further, the by-products of the foaming reaction can be physiologically damaging.
Additionally, use of mandrel dipping methods results in grafts which are limited to simple, thin-walled grafting material with reproducibility and uniformity being unattainable.
As will be described in greater detail hereinafter, the graft and method of the present invention have a number of advantages over the prior art grafts and methods, such advantages including a simple method of formation using a nonsolvent polymer system, ease of reproducibility of the exact graft structure, uniformity of the porous network within the graft while allowing for variable porosity and variable wall thickness of the graft, and the use of hydrophobic materials in the production of the graft.